In a typical wireless communications system, in order to reduce power consumption, a receiver periodically wakes up from a sleep state to determine if there is a transmission for which it is the intended receiver. If there is not, then the receiver can go back to sleep. Since the amount of power consumed while the receiver is in the sleep state can be much less than when the receiver is operating in a normal mode and the amount of time the receiver spends determining if there is an incoming transmission is short, the average power consumption can be quite low. The low power consumption may lead to extended periods between recharging (or replacing) the batteries used in the receiver.
However, while the receiver is on for the determination if there is an incoming transmission, a comparatively large amount of power may still be consumed. Therefore, it can be desirable to reduce the amount of time that the receiver is active. One technique that can be used to reduce the amount of on-time for the receiver is to use a short preamble. The short preamble reduces the amount of time that is required to receive the transmission, therefore reducing the on-time. Another technique that can be used involves the use of a specific and easy-to-detect preamble.
One disadvantage of the prior art is that by shortening the preamble, the amount of time that the transmission is on the air is shortened. By shortening the air-time, it may be easier for a receiver to miss the transmission, which can lead to the receiver missing an incoming call or message.
A second disadvantage of the prior art is that by shortening the preamble, existing advantages of a particular wireless communications technique may not be fully exploited, such as making full use of available bandwidth, exploiting the frequency diversity available in a communications channel, complying to technical and regulatory requirements, operating in the presence of interferers, and so forth.